Embodiments relate to a method of operating a nonvolatile memory device.
In recent years, there has been an increasing demand for nonvolatile memory devices which can be electrically programmed and erased and which do not require the refresh function of rewriting data at specific intervals.
The nonvolatile memory device is configured to enable electrical program and erase operations and perform the program and erase operations through threshold voltages which vary when electrons are moved by a strong electric field applied to a thin oxide layer.
The nonvolatile memory device mainly includes a memory cell array in which cells for storing data are arranged in a matrix form and a page buffer for writing data into specific cells of the memory cell array or reading data stored in specific cells thereof. The page buffer includes a bit line pair coupled to a specific memory cell, a register for temporarily storing data to be written into the memory cell array or for reading data stored in a specific memory cell of the memory cell array and temporarily storing the read data, a sense node for sensing the voltage level of a specific bit line or a specific register, and a bit line selection unit for controlling whether or not to couple the specific bit line to the sensing node.
It is known that the program speed of such a nonvolatile memory device becomes fast with an increase in the number of program/erase cycles. This is because, with the number of program/erase cycles increasing, the amount of electric charges trapped at the floating gate of each memory cell increases, and so the threshold voltage of the memory cell rises. Thus, although a relatively small number of program pulses according to the incremental step pulse program (ISPP) method are applied, the memory cell is programmed to have a threshold voltage higher than a target voltage.
In view of the characteristic, in the ISPP method, a method of applying a low program start voltage may be taken into consideration. If this method is used when the number of program/erase cycles is small, however, there is a concern in that the program time increases. This is because the number of pulses that have to be applied according to a lowering in the program start voltage is increased.